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A USB Grounding Adapter
My Mac Mini has no provisions for grounding and that is a problem. This cable solved the problem.
My Mac Mini has no provisions for grounding and that is a problem. This cable solved the problem.
Warning: This project should
only be attempted by persons familiar with AC wiring and
relevant construction techniques. An error in assembling
the USB Grounding Adapter can result in grave danger of
bodily injury, death, or damage to equipment. Please
take this warning seriously.
The USB grounding adapter is merely a very short USB extension cable and a very
short three prong AC extension cord with a wire to connect their ground connections to one-another.
My use of USB on my Mac Mini
is not typical of most. Beyond the typical computer
peripherals such as a printer, scanner, mouse and keyboard,
I often have it connected to a microcontroller programmer or
to breadboard being tested.
Several month ago my first Mac Mini stopped working for the last time. It was seven years old and was starting to show its age, so while the expense of replacement was not welcome, moving up to a newer model that was supported by the current version of the Mac OSX operating system and with twice the memory was welcomed.
Upon opening the box I was frustrated to see that there was no ground connection on the power cable. It seems that somebody at Apple can't make up his mind as to whether grounding is good or evil. I've seen the ground conductor come, go, come and go again.
To be fair to Apple, the typical user of a Mac Mini would not notice any difference between a grounded chassis and an ungrounded one because the user would rarely come in contact with the chassis -the outside of everything including the computer itself is insulated with plastic. Even if a user did come in contact with a conductive part of the computer, very strict safety requirements limit the maximum current that could pass through the user's body to an annoying sting or "buzz" as I have heard it called.
That leakage current is not good for the circuits on my workbench. The first sign of trouble was the destruction of an in-circuit programmer I had been using without trouble for 10 years. But with the ungrounded Mac, the programmer was "fried" beyond repair. Eventually I obtained a replacement for the programmer.
Not long after that, a microcontroller in a breadboard became damage, apparently from the same grounding issue and had to be discarded. Wary of this, I started to be more careful this time about the sequence of plugging things together.
Then a couple of weeks ago, I destroyed two more microcontrollers. Before switching to the unused Mac, destroying microcontrollers were ver rare events, and now it started to look like the problem was going to be very inconvenient, and a little bit expensive. I had procrastinated enough and did what I knew I would have to do as soon as I saw the ungrounded power cord: I made an adapter to ground the Mac Mini.
Several month ago my first Mac Mini stopped working for the last time. It was seven years old and was starting to show its age, so while the expense of replacement was not welcome, moving up to a newer model that was supported by the current version of the Mac OSX operating system and with twice the memory was welcomed.
Upon opening the box I was frustrated to see that there was no ground connection on the power cable. It seems that somebody at Apple can't make up his mind as to whether grounding is good or evil. I've seen the ground conductor come, go, come and go again.
To be fair to Apple, the typical user of a Mac Mini would not notice any difference between a grounded chassis and an ungrounded one because the user would rarely come in contact with the chassis -the outside of everything including the computer itself is insulated with plastic. Even if a user did come in contact with a conductive part of the computer, very strict safety requirements limit the maximum current that could pass through the user's body to an annoying sting or "buzz" as I have heard it called.
That leakage current is not good for the circuits on my workbench. The first sign of trouble was the destruction of an in-circuit programmer I had been using without trouble for 10 years. But with the ungrounded Mac, the programmer was "fried" beyond repair. Eventually I obtained a replacement for the programmer.
Not long after that, a microcontroller in a breadboard became damage, apparently from the same grounding issue and had to be discarded. Wary of this, I started to be more careful this time about the sequence of plugging things together.
Then a couple of weeks ago, I destroyed two more microcontrollers. Before switching to the unused Mac, destroying microcontrollers were ver rare events, and now it started to look like the problem was going to be very inconvenient, and a little bit expensive. I had procrastinated enough and did what I knew I would have to do as soon as I saw the ungrounded power cord: I made an adapter to ground the Mac Mini.
Pin 4 is ground, and the
wire color is supposed to
be black or blue. Best to check with an ohmmeter first.
be black or blue. Best to check with an ohmmeter first.
There are few places on the Mac Mini that give access to "ground" but the ground connection on the USB cable is one, and it was when using USB that I had the problems, so that seemed to be the logical place to make the ground connection, and also since it was the point at which the Mini's ground would meet other grounds, it seemed to be the place that would have the lowest chance of causing damage to the Mini's circuitry. Since the Mini only has four external USB ports, I used a short USB extension cable with the ground wire tapped so that I would not loose use of the port. The ground connection was brought out of the USB extension cable on the heavier green green wire seen in the photograph. The green ground wire is two meters of 0.5 square mm stranded copper wire.
The reason I shortened the USB extension cable was that I hoped to avoid more degradation of the high speed digital signals that might have occurred with a longer cable, but I really don't know whether the splices I had to add made the signals worse than they would have otherwise been or not.
I cut the female connector off a grounded power cord and pulled a three prong AC socket from my junk box, and made a very short extension cord and connected the ground wire from the USB extension cable to the ground connection on the male plug's ground pin.
In use, I plugged the computer into the female AC connector, plugged the USB cable into the computer, plugged the male AC connector into the wall, and it was done. The Mac Mini is grounded. I don't know what the internal grounding scheme looks like in the Mac Min, but at least the USB port is earthed.
If you decide to build this or something like this, go back and read the waring at the top of this article. Good luck and be careful.
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First posted in November, 2013
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